Murine immunoglobulins (Ig) have been classified into eight distinct Ig classes on the basis of their heavy chains, and genetic polymorphisms of four of these heavy chain genes have been previously described. In our recent studies we have detected two additional polymorphisms proposed to involve the micron and delta chain genes. Analysis of a wide range of myeloma proteins has also revealed the existence of several "variant" forms of Ig molecules. The general scope of these present studies falls into three areas: 1. at the molecular level, an immunochemical analysis of the allotypes of murine IgM, IgD, IgG2a and IgG2b; myeloma proteins of the IgG3 class, "variant" IgG2 molecules, 2. at the cellular level, a detailed analysis of the possible sequential activation of Ig heavy chain genes during the process of B cell differentiation; and an analysis of possible C region heavy chains genes expressed by sensitized T lymphocytes, and 3. at the genetic level, an immunogenetic analysis of factors controlling the production of antiallotype antibodies, the linkage of heavy chain genes particularly as studied in newly developed allotype congenic strains of mice, and the relative expression of alternate alleles at the IgM and IgG2 loci in specific immine responses to defined antigens. These three areas will be integrated into a broad picture of the chemical and genetic basis of murine Ig allotypes, their structure and cellular expression.